Graded initial moisture content paper cable



July 13 1965 oi G. GARNI-:R ETAL 3,194,871

GRADED INITIAL MOISTURE CONTENT PAPER CABLE Filed June 9, 1961 lll 'rE nptnnune F Av-Tanuevs United States Patent O 3,194,871 GRADED INITIAL MSTURE CNTENT PAPER CAELE f @scar G. Garner, Westiield, and Louis Meyerhotl?, Me-

tischen, NJ., assignors to General Cable Corporation,

New York, N Y., a corporation of New Jersey Filed June 9, 196i, Ser. No. 116,@87 4 Claims. (Cl. 174-25) This invention relates to paper insulated cables and, more particularly, to paper insulated cables in which the initial moisture content of the applied paper tape is controlled to produce an improved cable.

In paper insulated cables the insulation comprises a plurality of paper tapes applied in concentric helical wrappings over the conductor or cable core. Adjacent turns in each wrapping are laid so that the edges are arranged in nearly abutting relationship. Overlying helical wrappings are staggered relative to each other to overlap the abutting edges. The paper tape contains varying amounts of moisture related primarily to the ambient relative humidity at the installation location. After the tapes are applied to the thickness required for the application intended, the built-up Wall of insulation is dried in a vacuum oven.

In the vacuum oven, the combined action of heat and vacuum remove substantially all of the moisture from the paper insulation. Subsequently, insulating oils or compounds are introduced into the oven to saturate or impregnate the insulation.

The dimensions of the paper `tape vary with the moisture content thereof. That is, with increased moisture content the paper tapes will swell, increasing the dimensions of the tape by an amount dependent upon the moisture content thereof, which in turn depends essentially upon the relative humidity of the ambient atmosphere.

lf the initial moisture content of the tapes is not controlled during application thereof, a varying and uncontrolled amount of shrinkage of the paper within the insulation wall, and concomitant variation in the compactness of the insulation, will result during drying in a vacuum oven.

Since the electrical strength of the insulation increases with the overall density of the insulation, the variation of shrinkage during drying leads to a similar variation in the electrical strength of the insulation, References herein to the density of the insulation, or a layer thereof, mean that proportion of the total volume occupied by the insulation, or the layer, which is made up of paper tape, and do not refer to the density of a single paper k tape unless so specied.

The art has, in the past, stressed the value ot uniformity of compactness to ensure product uniformity. For this purpose, some cable manufacturers have controlled the relative humidity conditions in the rooms in which the paper taping heads are located so that the relative humidity therein ranges from about 4G to 60%. More recently, it has been proposed that the relative humidity at the paper taping head be controllably maintained at a lower relative humidity to further decrease the initial moisture content of the appliedpaper tapes and, thus, to increase the insulation compactness. n n

yThe decrease in the initial moisture content of the applied paper tape by control of the relative humidity will, of course, be eiiective to increase the density of the insulation. However, concomitant with the application of such drier tapes is a disproportionate increase in the length of drying time necessary to remove the residual lmoisture in the paper tape insulation. It has been found that, despite the lower initial moisture content of the paper tapes, it takes longer to dry the cable insulation in a vacuum oven than to dry cable insulation made with tape ot higher initial moisture content,

The increased time required to dry the cable insulation when drier tape is used can be attributed to the decreased shrinkage oi the drier tape in the vacuum oven. The major portion ot the water vapor escapes from the insulation core along a zigzag path through the spaces between adjacent turns kand layers of the tape. By starting with initially drier tape, the shrinkage is reduced, producing smaller spaces between the tapes. The smaller spaces introduce higher ow resistance to the water vapor and, thus, entails longer drying time. With a cable having a one-inch thick wall of insulation, the difference in drying time may be several days.

Further, uniformity of density in a radial direction is not essential since the electrical stress varies as an inverse function of the distance from the core.

It is, therefore, one object of this invention to provide an improved cable construction and method of manufacture thereof having high density insulation applied thereto without disproportionate increase in the required drying time.

lt is anotherobject of this invention to provide an improved cable construction in which the insulation density is varied in accordance with the electrical stress by using papers of varying initial moisture content.

lt isa further object of this invention to provide an improved method for the manufacture ot paper insulated cable in which the density ot the insulation and uniformity oi compactness is maintained without adversely increasing the drying time.

it is a still further object of this invention to provide an improved cable construction and an improved method of fabricating .such cable in which the layer of paper insulation adjacent the cable core is applied undercontrolled low moisture content and the superimposed layers are applied with controlled, but higher, moisture content.

As used herein: the term wrapping designates a single helically Wrapped paper insulating tape; the term layer designates a plurality of overlying wrappings composed oi paper tapes all having similar initial moisture content; and the term wall designates the entire body of paper insulation comprising two or more or such layers, the densities of the insulation in the several layers. difiering from each other, after evacuation, by reason of the fact that the initial moisture content of the paper in one layer was different from that in the other layer or layers.

In accordance with lthese objects, we have provided, in accordance with a preferred embodiment of this inventicn, a paper insulated cable in which the portion or layer of -the paper tapes applied directly on the cable core are applied'with initial low moisture content and in which overlying layers oi tape are applied with increased moisture content. ln such cable, the shrinkage during drying of the paper tapes with high initial moisture content causes relatively wide spaces between tapes to improve the speed of moisture removal during treatment of the tape in a vacuum oven. The insulation irnmediately adjacent the cable core is; however, maintained at a .higher density. The higher density adjacent the cable core provides the electrical strength necessary since this higher density is at the place of highest electrical stress in the cable insulation.

ln accordance with the method of this invention,.the cable is manufactured by assembling the cable core and passing it through a paper taping machine, comprising a plurality of taping heads.J maintained in an atmosphere of controlled low humidity. ri`he paper taping heads apply an inner layer otk helically wrapped tapes of low Si moisture content for slightly less than half the thickness of the wall required `for the application intended. The cable is then passed through a similar paper taping machine for application of the remainder of the required plurality of tapes which are applied as an outer layer of helical wrappings at a higher moisture content.

The cable is then placed in the usual vacuum oven for drying. Euring drying in the vacuum oven, the greater shrinkage of the paper tapes of the outer layer will open spaces therebetween to provide a path for re- Inoval of the moisture from the inner layer of drier tape Wrappings adjacent the core. Thus, the moisture is r..- moved from the tapes without significant increase in the time interval for removal. The cable fabricated in accordance with this method has improved insulation properties since the density of the applied paper tapes adjacent the core are subject to the highest electrical stress without disproportionate increase in drying time.

The invention may be more easily understood by reference to the following description taken in conjunction with the accompanying drawings, of which:

FIGURE 1 is a cross section of a paper insulated cable as initially fabricated in accordance with this invention;

FIGURE 2 is a cross sectional view of the cable shown in FIGURE 1 after drying of the paper insulation there- FIGURE 3 is a schematic illustration of a cable assembly operation illustrating the method of fabrication of the cable shown in FIGURES l and 2; and

PEG-URE 4 is a plot of moisture content versus temperature for various relative humidities in which the moisture, as a percent of dry paper weight, is plotted along the scale of ordinates and temperature in degrees Fahrenheit is plotted along the scale of abscissa.

In FIGURES l and 2 there is shown a cable comprising a cable core It) upon which is applied a plurality of overlying, concentric, helical wrappings of paper tape 12 having a controllably low moisture content. Gn top of these tapes are applied a plurality of paper tape wrappings 14 having a controllably higher moisture content.

When subject to drying in a vacuum oven, the outer layer of tapes I4 will shrink more than the inner layer of ltapes f2 to provide a relatively wide path between tape layers and to a lesser extent, between adjacent tapes. The spaces opened by such shrinkage will provide a perambulating path indicated by dotted line i8 through the paper wrappings 14 of the outer layer to the inner drier tapes ft2. The inner tapes 12 applied with a lower initial moisture content will, of course, not shrink to the same extreme and will provide a more compact and uniform layer of insulation. However, the paths thereto through the layer of outer tapes will increase the drying effect of the oven and prevent the great increase in drying time that will 'result if the entire insulating wall is formed of the drier tapes.

For understanding of the invention, it shall be necessary to specifically study the dimensional changes of paper under varying degrees of moisture content and the mechanism of drying the paper in an applied wall of paper insulation.

It is known that the dimensions of the paper tape are affected by its moisture content. For example, conventional insulating tape having a moisture content of 7% (taken as a percentage of the dry weight of paper) is about 1% longer, 2% wider and 4% thicker than fully dried paper.

The moisture content of the paper will vary with the relative humidity and the temperature of the atmosphere as is shown in the graph of FIGURE 4, assuming equilib- Iium conditions have been reached. It will be noted that the temperature eifect is much smaller than that of the relative humidity. It will further be noted that changes in the moisture content of the paper tape are not directly proportional to changes in atmospheric relative humidity. For example, change of the relative humidity from 60 to 30% decreases the moisture content from 7.8% to 4.5% at temperature. Further, although control of the moisture content of the applied tape can, to some extent, be afforded by control of the relative humidity of the room in which the paper tapes are applied, it is still necessary to dry the tape in a vacuum oven to obtain the necessary dryness for paperinsulated oil-impregnated cable.

In the vacuum oven the temperature is raised to approximately C. The pressure ditlerence between the vapor pressure of the moisture within the insulation and the vacuum maintained in the oven forces the moisture out into the oven from which it is removed by a vacuum pump. Initially, the vapor pressure is high and drying proceeds relatively rapidly. As the paper becomes drier, the vapor pressure becomes correspondingly lower and the rate of drying decreases. Thus, while it is relatively easy to remove the large bulk of the moisture, removal of tie last few tenths of a percent is ditiicult and requires extended drying treatment.

The removal of the moisture from the interior of the plurality of applied paper tapes proceeds primarily through the zigzag paths provided by the butt spaces of adjacent tape turns in each wrapping and the spaces between overlying `concentric tape wrappings. Therefore, the larger the spaces between the tapes, the lower is the iiow resistance, and therefore the faster i-s the rate of drying. Conversely, smaller spaces inhibit the flow of the vapor and increase the drying time.

The increase in drying time is a drawback to the use of initially dry tapes in the fabrication of paper insulated cable. Although there is less moisture to remove from applied dry tape, the reduced shrinkage of such tape greatly increases the time necessary for drying the cable within the vacuum ovens. For example, if initially drier tapes (such as those applied under a controlled relative humidity of 10%) are used for the fabrication of a cable having a one-inch thick wall of paper insulation (eg. for 345 kv. cable), the drying time will increase by as much as several days. This increase in drying time renders such cable uneconomical in conventional practice, despite the apparent advantages of higher insulation density and an increased uniformity of insulation.

It will be noted, however, that a concentrically insulated cable is essentially a cylindrical condenser and the electrical stress throughout the insulation is inversely proportional to the diameter. For example, if the conductor diameter is 1.7" and the outer diameter of the insulation is 3.7, the electrical stress in the insulation at the conductor is 2.18 times the stress at the outer-most part of the insulation and is 1.37 times the stress at the mid-point of the insulation.

Therefore, the specific electrical strength of the insulation must be higher adjacent the cable core than is necessary at the outer diameter of the `insulation wall.

The increased electrical strength of the insulation may be provided by increased density of the applied paper tape insulation. The increased density of paper tape insulation adjacent the core can be attained without substantially increasing the drying time of the entire paper insulation by the use of relatively dry paper in the inner portion of the insulation and of less dry paper in the outer portion as illustrated in FIGURES 1 and 2. The use of high density paper insulation is unnecessary over the entire wall.

During drying of such cable, the tapes of the outer layer will shrink providing increased path for moisture flow from the inner layer of drier paper tapes. Thus, there is afforded an inner layer of high density paper tape insulation with an overlying layer of paper tape insulation having a lower electrical strength, but positioned at a point of lower electrical stress.

The method of fabricating paper insulated cable in accordance with this invention is best seen by reference to FIGURE 3.

In FEGURE 3 there is 'shown the cable core 10 which is passed through a paper taping machine having a plurality of paper taping heads 2t). Although only one paper taping head is illustrated for simplicity, a conventional taping machine may have twelve taping heads, each of which is provided with ten pad holders 22. Thus a total ot 120 tapes 24- are applied to the core as it passes through the paper taping machine.

The paper taping machine is maintained in an enclosure defined by walls 25. The relative humidity within the enclosure is maintained low as, vfor example, at 10% RH. The paper tapes are allowed to reach equilibrium condition with the relative humidity of the room. While this might be done by storing the tape in the controlled iroom, it is usual to dry the tapes to the approximate moisture content desired by passing the tape over heated rolls. The mechanical drying of the tapes to` approximately the equilibrium condition is much faster and usually found advisable.

As can be seen from FIGURE 4, at 10% relative humidity, the tapes will reach equilibrium at about 3% relative moisture. The tapes of initially low moisture content are applied until the insulation thickness is somewhat less than one half of the desired insulation wall thickness. For the example taken, 12() tape wraps are applied at this stage. y

The cable is then fed through a second paper taping machine having, for example, 14 paper taping heads 28 each of which has ten tape pads 30. Thus 140 tapes 32 are applied by the machine. The relative humidity of the enclosure 34 may be maintained relatively higher, for example, 30% RH., so that the outer tapes are applied at a relatively higher initial moisture content of 4 to 4.5% determinable from FTGURE 4.

Thus the entire cable insulation is built up from a plurality of relatively dry tape wrappings adjacent the core over which is applied a plurality of Irelatively moist tape wrappings. On drying of the cable insulation, the difference in the ishringage of the tapes with different initial moisture content will result in a relatively high density insulation adjacent the core (i.e, at the position of highest electrical stress) and a relatively lower den-sity insulation over the high density insulation.

This invention may be variously ymodified and embodied within the scope of the Isubjoined claims.

What is claimed is:

1. An electric cable comprising a conductor and an enclosing wall of paper insulation made up of at least two layers each consisting of a plurality of overly-ing helical wrappings of paper tapes, the inner layer having the overlying wrappings in closer relation axially and radially to one another than are the wrappings of the outer layer, the overlying wrappings of the outer layer being in a looser relation to one another with greater radial lspacings between the overlying wrappings than the spacings between the overlying wrappings ofthe inner layer, said ispacings in each layer being distributed throughout the circumferential extent and radial thickness ofthe layer.

2. The electric cable described in claim 1 characterized by the 'paper tapes in the inner and outerlayers being similar and the individual wrappings having the paper tapes wound with the edges in successive convolutions in nearly abutting relationship to provide axial spacing of convolutions, and said radial spacings in each layer being substantially uniform throughout the layer.

3. The electric cable described in claim 1 characterized by the inner layer being of less radial thickness than the outer layer.

4. An electric cable comprising a conductor and an enclosing wall of paper insulation made up of at least two layers each consisting of a plurality of overlying helical wrappings of similar paper tapes, the spaces between wrappings and between the turns thereof in the inner layer being less than the spaces in the overlying layer, the spaces in each layer being uniform throughout the layer.

References Cited by the Examiner UNiTED STATES PATENTS 2,097,501 11/37 Reichel-t 156-54 2,102,974 12/37 Robinson 174-25 2,190,017 2/40 Del Mar 174-26 2,195,998 4/40 Race 174-122 2,770,565 11/56 Pergunas et al. 156-54 2,827,510 3/58 Hunsinger 174-122 2,831,050 4/58 Mulligan 174-122 JOHN F. BURNS, Primary Examiner.

EARL M. BERGERT, OHN P. WILDMAN,

Examiners- 

1. AN ELECTRIC CABLE COMPRISING A CONDUCTOR AND AN ENCLOSING WALL OF PAPER INSULATION MADE UP OF AT LEAST TWO LAYERS EACH CONSISTING OF A PLURALITY OF OVERLYING HELICAL WRAPPINGS OF PAPER TAPES, THE INNER LAYER HAVING THE OVERLYING WRAPPINGS IN CLOSER RELATION AXIALLY AND RADIALLY TO ONE ANOTHER THAN ARE THE WRAPPINGS OF THE OUTER LAYER, THE OVERLYING WRAPPINGS OF THE OUTER LAYER BEING IN A LOOSER RELATION TO ONE ANOTHER WITH GREATER RADIAL SPACINGS BETWEEN THE OVERLYING WRAPPINGS THAN THE SPACINGS BETWEEN THE OVERLYING WRAPPINGS OF THE INNER LAYER, SAID SPACINGS IN EACH LAYER BEING DISTRIBUTED THROUGHOUT THE CIRCUMFERENTIAL EXTENT AND RADICAL THICKNESS OF THE LAYER. 